1. Field of the Invention
The instant invention is directed to a catalyst useful in the polymerization of propylene. More particularly, the instant invention is directed to a catalyst, useful in the polymerization of propylene, wherein a silica support is modified and thereafter treated with magnesium and titanium compounds.
2. Background of the Prior Art
The polymerization of olefins using Ziegler-Natta catalysts is widely utilized. These catalysts provide polyolefins in high yield possessing the desired characteristics of these polymers. However, the use of these conventional catalysts are subject to important failings. Thus, new and improved catalysts are continually being sought. An important class of catalysts where improvement is sought is the very commercially important alpha-olefin, propylene.
Commonly in the polymerization of propylene a catalyst having a magnesium halide support is utilized. However, when polypropylene polymerized from a magnesium halide supported catalyst is processed into molded products, the polypropylene molding machine processing this polymer is subjected to corrosion. This corrosion is caused by the residual presence of magnesium halide in the polypropylene product. The adverse effect of this corrosion is not limited to damaging expensive molding machinery. More importantly, the polypropylene molded article processed in this equipment is characterized by aesthetic flaws.
Another detrimental property of catalysts, conventionally used in the polymerization of propylene polymers, involves the characteristic in many propylene catalysts of the prior art that they incorporate internal electron donors to insure that the propylene polymer product is highly isotactic. Those skilled in the art are aware of the criticality of stereoregularity in propylene polymers. However, those skilled in the art are also aware that the presence of internal electron donors creates difficulties. Unless the amount and type of electron donor compound is carefully selected not only is the stereoregularity of the resultant polymer deficient but poor catalytic activity often results. This detrimental effect occurs even if the amount and type of electron donor is properly chosen if the catalyst is formed with the electron donor compound added in the wrong sequence.
The utilization of electron donor compounds often creates additional problems involving offensive odors in the final polymeric product. This unfortunate result obtains even if the ideal electron donor compound, in the correct concentration, added at the proper time in the catalyst formation process, is utilized. Thus, polymers formed from catalysts which include an electron donor compound must oftentimes be deashed or deodorized in order to insure that the final product gives off no odor.
The difficulties discussed above have spurred workers skilled in this art to develop new catalysts which attempt to overcome these difficulties. In one such attempt at eliminating the problem created by halogen-containing carriers, inorganic oxides, such as silica, were proposed as a support. This carrier, containing no halogen, was reacted with a magnesium dialkoxide and an electron donor, such as a carboxylic acid monoester, and a titanium halide compound. This catalyst is described in Japanese Patent Publication 62,607/1983.
Even if the allegations made in this disclosure of high catalytic activity, production of a highly stereospecific polymer having a high bulk density and narrow particle size distribution were correct, still the problems associated with odor are not addressed by the use of this catalyst. However, testing of this catalyst establishes that the catalyst provides low activity and that the polymer product is characterized by insufficient stereoregularity and poor particle size distribution.
A more recent disclosure, U.S. Pat. No. 4,595,735, provides a catalyst component for the polymerization of olefins prepared by contacting a magnesium alkoxide, a halogenated hydrocarbon, a halogenated silane and a titanium compound. It is emphasized that this catalyst, useful in the polymerization of ethylene homopolymers and copolymers, incorporates a halogenated hydrocarbon. This catalyst is not only principally directed to the polymerization of ethylene polymers but, significantly, emphasizes the formation of high melt index polymers. Those skilled in the art are aware of the necessity for commercially useful polypropylene to possess low melt flow rates. That is, the molecular weight of the polymers produced in accordance with the '735 catalyst is significantly lower than that required of polypropylene.
U.S. Pat. No. 4,565,795 sets forth an olefin polymerization catalyst which is prepared by the reaction of a chemically treated silica support with a dihydrocarbyl magnesium compound and a halogenated tetravalent titanium compound. The chemical treatment of the silica support involves the use of a chlorinating compound, an alkanol, a silating compound, an acid chloride or an organoboron compound. Again, this catalyst includes constituents which are adverse to the production of stereoregular polymers, especially polypropylene. It is thus not surprising that this catalyst is suggested for use in the polymerization of ethylene polymers.
The above remarks make clear the continuing need in the art for a new polypropylene catalyst having the desirable properties enumerated above. They also establish that the recent prior art has not met that need.